I know that they're hard to find parts for, but sure are fun!! My buddy gave his away for $20 out of frustration from not being able to find interchangeable parts.
These have less than 1" axle, right? Aside from the weird hub mounting, I assume the axle bearing hangers are standard 2 or 3 hole?
You would probably have to replace the whole rear end: axle, bearings, hubs, wheels, spacers, brake disc/hub, sprocket/hub, etc. to move to a 1" axle and have some more options like bigger tires and so forth.
If you work on the motor and gearing, that thing will hum!!